The present invention relates to a power semiconductor device, in particular to a fast switching diode.
When a pn-diode is conducting a forward current, electrons and holes are injected from the highly doped n+ and p+ surface layers into the n− layer. These injected carriers increase the concentration of mobile charges and thereby increase the conductivity of the n− layer. This conductivity modulation is necessary to reduce the voltage drop of a diode in the forward conducting mode.
When the applied voltage is reversed, the diode has to block the current flow. A depletion layer spreads from the pn-junction into the n− layer, building up an electric field, which supports the reverse voltage.
The transition between forward conducting and reverse blocking mode has a certain time delay during which time a short reverse current peak is flowing. These characteristics are specified with the parameters “Reverse Recovery Time” trr, “Reverse Recovery Current” Irrm and “Reverse Recovery Charge” Qrr. For fast recovery diodes all these parameters are expected to be as small as possible. However, a fast switching capability of a diode is generally accompanied with an increased forward voltage drop Vf, so that one has to find a compromise for a specific application of the diode.
The switching speed of a pn-diode can be increased (i.e., trr, Irr and Qrr can be reduced) by introducing recombination centers into the silicon crystal, where the injected electrons and holes can recombine. Recombination centers are generally defined using heavy-metal atoms, such as gold or platinum. Sometimes crystal defects are created within the substrate for this purpose. The crystal defects are created by irradiating the substrate with high energy particles, e.g., electrons or helium atoms. The higher the concentration of recombination centers, the smaller is the carrier lifetime and consequently the trr.